Coating compositions comprising a pigment and binder are generally employed in the manufacture of paper in order to improve its printing properties, optical characteristics and appearance. It is well known that a paper coating composition must have certain characteristics in order to perform these functions; in particular, it must have the proper viscosity and rheological characteristics to permit its application to the paper by modern high-speed machines and to spread properly on the paper. Moreover, the binder, which serves to bind the pigment and to adhere the coating to the paper surface, must be such that it will provide a uniform, homogeneous coating film that will withstand the stresses encountered during subsequent printing and/or converting operations.
In utilizing paper coating compositions, it is most desired that the coatings, once applied, will be rapidly immobilized on the paper web surface. Such rapid immobilization results in improved fiber coverage, decreased coating densification and minimized binder migration. These coating structural effects then provide potential benefits such as improved fiber covering power, increased opacification, smoother surface and better printing characteristics on the final coated paper substrate.
Previous attempts to achieve rapid immobilization of paper coating compositions involved the use of cationic starches and proteins to produce partially flocculated coatings which gained viscosity rapidly upon the solids increase that occurred subsequent to the coating process. However, these approaches were not totally satisfactory and found limited application since they often produced paper coatings with unacceptable rheological characteristics.